Cathode ray tube with X-ray absorbing coating



July 28, 1942, H. w. PAEHR I CATHODE RAY TUBE WITH X-RAY ABSORBINGCOATING Filed Nov. 1, 1939 INVENTOR/ TTORNEYY" Patented July 28, 1942CATHODE RAY TUBE WITH X-RAY ABSORB- ING COATING Application November 1,1939, Serial No. 302,401

In Germany November 3, 1938 Claims.

The invention relates to cathode ray tubes, particularly for televisionand oscillograph purposes, using anode potentials of high voltage sothat the production of X-rays must be taken into consideration.

It is an object of the present invention to provide a cathode ray tubepa y o he p jection type which, in spite of the high voltage used foraccelerating the electrons impinging upon the luminescent screen of thetube, can safely be used with regard to X-rays produced by the electronsimpinging upon the screen. It is a further object to provide a tubehaving an envelope which effectively prevents the passage of X-rays tothe outside of the tube in order to avoid detrimental influences on thehealth of persons operating such tubes. It is known that the repeatedinfluence of even small doses of X-rays over a long period is sufiicientto produce detrimental efiects and may lead to cancer and other seriousillness.

A further object is to provide a tube which is so constructed that thenecessity of bulky and heavy external X-ray screens is avoided. SuchX-ray screens consist, for instance, of lead in the form of plates,sheets or foils. Also, a use of leadrubber and of so-called Kampe-Loreysbarytstones has been suggested. All these substances, however, can onlybe used on the outside of high vacuum tubes.

It is a further object to avoid the drawbacks which result if the wholetube envelope is made of lead-glass. This glass is not suited forblowing the complete cathode ray tube. Only that part of the tube whichmust be transparent in order to allow the passage of light out of thetube is made of lead-glass.

According to the invention the envelope of the cathode ray tube isprovided with an inner coating consisting of a material having a dark,pref erably black non-reflecting surface and being impervious to theX-rays produced by the cathode-ray impacting the light-emitting screenof the tube. The material is of such a nature that it acts at the sametime as an electrical shield against electrostatic fields so that wallcharges are avoided. A coating of this type has the advantage that noadditional shielding or screening tions of light into the interior ofthe tube are avoided. The coating consists, for instance, of saltsand/or oxides of heavy metals, for instance lead-sulfide (PbS) bismuthsulfide (BizSs) or bismuth-dioxide (B102).

The coating may either consist entirely of the X-ray absorbing materialor it may consist of a mixture contain ng a binder with adhesiveproperties, the X-ray absorbing material, and, if required, a blackeningagent, for instance finely divided carbon. According to the compositionof the wall coating, the method of applying it may be varied as will bedescribed in connection with the drawing.

The drawing shows by way of example a cross section through a tube madein accordance with the invention.

The cathode-ray tube l contains an electron gun 2 and a luminescentscreen 3. The inner wall of the tube is covered with a coating 4 servingas an electrical shield, having light-absorbing properties and absorbingX-rays coming from the screen. The end wall 5 of the tube is free fromthe coating material because the light produced by the screen 3 mustpass through the wall of the tube. This window 5 is therefore preferablymade of lead-glass of sufficient thickness to avoid the passage ofX-rays. Also, the part of the tube Wall near the electron gun is freefrom coating because this part is shielded by the anode 6 against thepassage of X-rays.

Measurements have shown that a cathode ray tube with a ray current of0.5 mA. and 20 kV. screen potential would require a lead screen ofthickness of 0.03 mm. The corresponding value for 30 kV. screen voltageis 0.14 mm., and for kV. 1.31 mm. This shows that particularly for themedium voltage as, for instance, used in home projection receivers, acoating in the order of 0.1 to 1.0 mm. thickness will suffice.

The coating may be produced in a number of different ways. In case itconsists solely of a metal salt it may be felled out of a solution in asimilar manner as silver coatings are made.

Another method is the following: The parts of the envelope which shallremain free of the coating are shielded by suitable screens arrangedwithin the tube. Then a portion of lead is introduced into the tube andevaporated by heat. The vaporized metal covers the inner surface of theenvelope and is then treated by an electrolytic process in order to giveto the surface non-reflecting properties. The coating is oxidizedanodically so that a surface of lead superoxide is formed. It is alsopossible to use a solution containing phosphorous acids for thispurpose.

Another method is to produce the coating completely by electrolysis. Inthis case it will be preferable to pretreat the glass wall of the tubeby producing thereon a thin conducting layer and to deposit on thislayer the X-ray absorbing material.

Another way is to mix the X-ray absorbing salt or oxide in finelydivided form with an adhesive obtained which can be applied to the tubewall I in a similar manner as colloidal graphite. It may be preferableto mix this material with colloidal graphite in order to obtain thedesired conductivity and. non-reflecting properties. PbS does not meltand sublimise below 600 C.

What I claim is:

1. In a cathode ray tube comprising an evacu ated envelope havingtherein an electron gun and a luminescent screen adapted to be impactedby cathode rays, a coating on the inside of the wall of said envelope,said coating consisting of a nonrefiecting material impervious to X-raysproduced by the cathode rays impacting said luminescent screen, saidwall coating covering substantially all of said. envelope between saidgun and said screen and forming a shield against electrostatic fields.

2. In a cathode ray tube comprising an evacuated envelope having thereinan electron gun and a luminescent screen, a coating on the inside of thewall of said envelope, said coating covering substantially all of saidenvelope between said gun and said screen and consisting of anelectrically conducting material which is substantially impervious toX-rays produced by the impact of cathode rays upon said screen andhaving a black non-reflecting inner surface.

3. In a, cathode ray tube comprising an evacuated envelope havingtherein an electron gun and a light emitting screen adapted to beimpacted by cathode rays, a coating on the inside of said envelopeconsisting at least partly of leadsulfide, said coating coveringsubstantially all of said envelope between said gun and said screen.

4. In a cathode ray tube for television purposes comprising an evacuatedenvelope having therein an electron gun and a light emitting screenadapted to be impinged. by the cathode ray, a light transparent windowin said tube, said windew consisting of lead-glass, a coating on theinside of the wall of the envelope between said gun and said screen,said coating consisting of a compound of a heavy metal of suificientthickness to absorb the X-rays produced on said light emitting screenand having a black non-reflectinginner surface.

5. In a cathode ray tube comprising an evacuated envelope having thereinan electron gun and a light emitting screen, a coating on the inside ofthe wall of the envelope comprising a mixture including an adhesivebinder, a blackening agent and an X-ray absorbing agent, said coatingcovering substantially all of said envelope between said gun and saidscreen.

HANS WERNER PAEHR.

